Hierarchical Nanopatters by Nanoimprint Lithography

ABSTRACT

A method for forming hierarchical patterns on an article by nanoimprinting is disclosed. The method includes using a first mould to form a primary pattern on the article at a first temperature and a first pressure, the first temperature and the first pressure being able to reduce the elastic modulus of the article; and using a second mould to form a second pattern on the primary pattern at a second temperature that is below the article&#39;s glass transition temperature, the forming of the second pattern being at a second pressure.

FIELD OF THE INVENTION

This invention relates a method forming hierarchical patterns bynanoimprinting and relates particularly though not exclusively to amethod for forming ordered three dimensional (3-D) hierarchicalstructures.

BACKGROUND OF THE INVENTION

Development of hierarchical or three-dimensional structures at thesub-micrometer scale is becoming increasingly important withtechnological advances in microelectromechanical andnanoelectromechanical systems, microfluidic devices, microoptics ornano-optics, toolsets for biologists (microfluidic chips for deoxyribosenucleic acid array), and medicine (microsurgical tools).

Hierarchical structures are responsible for some unique properties inthe natural world such as the extreme surface hydrophobicity of a lotusleaf, the super water-repellency of a water strider's leg, and thereduced water resistance of a shark's skin. Man-made applicationsinclude the application of a plastic coating with hierarchical surfacetopology on aircraft for drag reduction.

Currently available fabrication techniques include micro stereolithography, a combination process of deep reactive ion etching and bulkmicromachining, inclined deep X-ray lithography, and inclinedultraviolet lithography. All have demonstrated three dimensionalmicrostructures and nanostructures. However, fabrication of hierarchicalstructures has not been possible. Most hierarchical structures reportedwere obtained from a self-assembly method. However, the fidelity of suchstructures and their long-range order are poor. These techniques arealso low in throughput.

Since the publication by Chou et. al Stephen Y. Chou, Peter R. Krauss,Preston J. Renstrom, Appl. Phys. Lett 67 (1995) 3114, nanoimprintlithography (“NIL”) has been recognized as an attractive techniqueparticularly for fabrication of two dimensional nanostructures. Theprimary working principle of nanoimprint lithography relies on theviscoelastic properties of polymers. In this way, a polymer film isheated to above its glass transition temperature (Tg). The polymer willthen flow and acquire the topology of a hard mould. The pattern is setwhen the polymer is cooled to its glassy state. Therefore, the patternresolution is primarily determined by the hard mould.

SUMMARY OF THE INVENTION

In accordance with a first preferred aspect there is provided a methodfor forming hierarchical patterns on an article by nanoimprinting, themethod comprising:

(a) using a first mould to form a primary pattern on the article at afirst temperature and a first pressure, the first temperature and thefirst pressure being able to reduce the elastic modulus of the article;

(b) using a second mould to form a second pattern on the primary patternat a second temperature that is below the article's glass transitiontemperature, the forming of the second pattern being at a secondpressure.

The first temperature and pressure may be also able to reduce the meltviscosity of the article. The first temperature is preferably above theglass transition temperature of the article. The article may be apolymer film, and may be one or more of: polycarbonate,polymethylmethacrylate, and a polyimide with hydroxyl side groups. Thepolymer film may be a polymer composite reinforced with particlesselected from: calcium carbonate, carbon filler, glass filler, fibers,glass fibers, carbon fibers, and carbon nanotubes. The polymer film maybe on a substrate.

The secondary pattern may be formed at an angle relative to the primarypattern, the angle being in the range 0° to 90°.

At least one of the first mould and the second mould may be treated witha low surface energy coating for a facilitating their release from thearticle. The coating may be perfluorodecyltrichlorosilane.

The first mould may have a first grating for forming a first gratingstructure on the article, and the second mould may have second gratingfor forming a second grating structure on the first grating structure.The first grating may be larger than the second grating. Alternatively,the first grating may be smaller than or may be the same as the secondgrating. The first grating structure may be protrusions and trenches onthe article. The second grating may be formed on the protrusions and inthe trenches. The first grating may be 2 micrometers and the secondgrating may be 250 nanometers. The first grating and the second gratingmay have different geometries.

Preferably, the first temperature is in the range of 120° C. to 200° C.;the secondary temperature is in the range of 60° C. to 200° C.; theprimary pressure is in the range of 40 bar to 50 bar; and the secondarypressure is in the range of 10 bar to 50 bar.

More preferably, residual solvent is removed from the article. Thepolyimide with hydroxyl side groups may be a porous film.

According to a further preferred aspect there is included a productproduced by the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be fully understood and readilyput into practical effect, there shall now be described by way ofnon-limitative example only preferred embodiments of the presentinvention, the description being with reference to the accompanyingillustrative drawings.

In the drawings:

FIG. 1 is a schematic illustration of the method for hierarchicalnanoimprinting;

FIG. 2 is a series of SEM images of patterns in a freestanding PC film;

FIG. 3 is a series of SEM images of patterns on a substrate-supported PCfilm;

FIG. 4 is a series of SEM images of patterns on PMMA films spun ontosilicon substrates;

FIG. 5 is a graph of correlation between the secondary imprint depth andimprint temperature (for PMMA spin coated films);

FIG. 6 is a series of SEM images of patterns on HPI spin-coated film;

FIG. 7 is two graphs of load and displacement nanoindentation curves;

FIG. 8 is an illustration of tapping-mode AFM images; and

FIG. 9 is an illustration for polymer flow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process for fabrication of hierarchical nanostructure isschematically shown in FIG. 1. A first mould 10 having a first grating14 that comprises protrusions 11 and trenches 13 is first pressedagainst a polymer film 12, either free standing or supported on asubstrate 15. This is preferably at a temperature above the glasstransition temperature, Tg, of the polymer film 12. The pressure used ispredetermined. The combination of the pressure and temperature should besufficient to reduce the modulus of the film 12. Preferably, thetemperature and pressure are sufficient to reduce the elastic modulusand the elastic viscosity of the film 12. Upon demoulding, a primarypattern 16 of protrusions 17 and trenches 19 is created in the polymerfilm 12. In the next step, a second mould 18 having a second grating 20is used. The second grating 20 may be different to the first grating 14,or may be the same as the first grating 14. The second mould 18 is thenis aligned at a predetermined orientation relative to the polymer film12 and pressed against the pre-patterned polymer film 12 at atemperature below Tg, preferably well below Tg, to create a secondarypattern over, but without destroying, the first pattern 16. Likewise, atertiary or higher-order pattern can be progressively produced. Thesecond grating 20 may be a larger number of closely-packed spikes 22.The first grating 14 and the second grating 20 may be of differentgeometries, if required or desired. They may also be of sizes that aredifferent, or may be the same.

In the above process, the use of an imprinting temperature below Tg inthe secondary or higher-order imprints is to insure that the primarypattern is not destroyed during the subsequent imprints.

By varying the alignment between the two imprinting steps andcontrolling the imprinting conditions for the subsequent imprinting, avariety of sophisticated hierarchical patterns may be obtained.Imprinting recipes may be optimized so that deformation of the primarypattern 16 is minimized. The imprinting temperature for the second andsubsequent imprinting should be at a temperature lower than Tg of thepolymer 12. Subsequent imprinting pressure and time duration also have asignificant influence on the final feature of the resultant hierarchicalstructure.

Suitable polymer films 12 include films of different polymers such asfor example, polycarbonate (PC), polymethylmethacrylate (“PMMA”) and ansynthesized polyimide with hydroxyl side groups (HPI). Their Tg's are150° C., 105° C. and 415° C., respectively. The polymer film 12 may alsobe a polymer composite reinforced with particles such as, for example,calcium carbonate, carbon filler, glass filler, fibers, glass fibers,carbon fibers, and carbon nanotubes.

Example 1

In this example is described a series of hierarchical imprinting onfreestanding films of polycarbonate (“PC”), Tg=150° C. A primary patternis first imprinted on the films using a 2 μm grating 14 mould 10 withthe grating 14 being in SiO₂ and being treated with oxygen plasma. Theimprinting is for 5 minutes at 180° and 40 bar and results in a 2 μmgrating structure 16 on the film 12. The secondary imprint issubsequently carried out using a 250 nm grating 10 mould 18 in SiO₂ alsotreated with oxygen plasma. The first mould 10 has a grating 14 that islarger than the grating 18 of the second mould 18.

The hierarchical structure depends strongly on the imprinting recipe andthe alignment of mould 18 in the secondary and subsequent imprints. Whenthe secondary imprint is carried out at 100° C. (50° C. below Tg) and 49bar, a 250 nm grating structure is created over the entire primarygrating structure, i.e. both the protrusions 17 and the trench 19 areimprinted. The resulting structure is shown in FIG. 2A. In this example,the secondary imprint is at a perpendicular (i.e. 90°) alignment withrespect to the primary pattern. With the same imprinting temperature andpressure, different hierarchical nanostructures are obtained when 45° orparallel (i.e. 0°) alignment is employed (FIGS. 2B and 2C respectively).

By lowering the second imprinting temperature to 80° C. and at apressure of 40 bar, the 250 nm imprint is created in the protrusion ofthe primary pattern (FIG. 2D); but at 80° C. and 10 bar, only a slightimprint is observed at the trench bottom (FIG. 2E). Further lowering theimprinting temperature to 70° C. (more than 50% below Tg) and thepressure to 10 bar, virtually no imprint is observed at the trenchbottom (FIG. 2F). These worked only on a pre-imprinted polymer. Whenused on a flat film, imprinting did not occur.

The second imprint cuts into the first imprint over the entire gratingstructure 16 and may result in some flattening of the protrusions 17 dueto the applied pressure, but this does not undermine the hierarchicalstructure.

As seen from the above description, nanoimprinting occurs inpre-patterned PC films at temperatures as low as 100° C. to 70° C. (50°C.-80° C. below the Tg of the polymer). Nanoimprinting occurs becausethe polymer becomes softer after the primary imprinting. The polymerbecomes softer because the elastic modulus of the polymer is lower.Also, the melt viscosity of the polymer may be lower.

Example 2

Hierarchical nanoimprinting is conducted on substrate-supported thinfilms. In this example, either or both moulds 10, 18 are treated with alow-surface-energy perfluorodecyltrichlorosilane (“FDTS”) to facilitatetheir release from the films after imprinting. The silane treatmentallows a higher yield of patterning, without which the polymer film mayadhere to the mould 10, 18 during demoulding.

A 5% wt PC solution in tetrahydrofuran (THF) is spin coated onto a baresilicon wafer sonicated in heptane or isopropanol and then rinsed withacetone, without plasma treatment, and blown dry with compressednitrogen. Spinning is at 6000 rpm for 30 seconds to give a film withthickness of about 400 nm. The film is then baked in air at 80° C. for 5min to remove any residual solvent. Next, the film is imprinted withmould 10 (O₂ plasma and FDTS treated) at 160° C., 40 bar for 2 min andthen separated. Mould 18 (O₂ plasma and FDTS treated) is subsequentlyaligned in parallel patterns and pressed at 80° C. and 15 bar for 30 s.By doing so, protrusions of 2 micron grating are lengthwise “sliced” bythe 250 nm grating, while the trench bottoms remain intact (FIG. 3-A).When the resultant structure is further imprinted by mould 18 (alignedvertically to the secondary pattern), a tertiary pattern results andgives rise to the formation of nano-squares on the protrusions of the 2micron grating (FIG. 3-B).

In both the two-step and three-step sequences, the 250 nm secondary andtertiary gratings are created at 80° C. and 15 bar.

Example 3

In this example, films of 400 nm thick polymethylmethacrylate (PMMA,Tg=105° C.) are imprinted hierarchically. The film is prepared in thesame way as the PC of examples 1 and 2, except that the solvent used istoluene. Imprinting consists of 120° C., 40 bar for 120 s for the firststep (with FDTS treated mould 10) and 60° C.-70° C., 40 bar for 120 sfor the second step (with FDTS treated mould 18 aligned vertically orhorizontally to the previous grating). SEM images of the differentpatterns created are shown in FIG. 4, which shows a secondary 250 nmgrating with good resolution on the 2 micron protrusions.

To further illustrate this embodiment, an AFM line scan was used tomeasure the depth of the second imprint. This used a spin coated PMMAfilm supported on an Si substrate. FIG. 5 shows that the imprint depthis controlled by the imprinting temperature. As seen in FIG. 5, thesecondary imprinting temperature was between 90 degrees C. and 50degrees C., and the imprint depth varies from about 200 nm to 80 nm froma 250 nm deep grating mould. The temperature range is below the Tg ofthe PMMA film. At these temperatures, imprinting may not occur on a flator a spin coated PMMA film.

Hierarchical imprinting may also be achieved when a porous material isused. A synthesized HPI (a polyimide with hydroxyl side groups)demonstrates this. A 2.5% (by weight) HPI solution in THF is spun onto aSi substrate at 5500 rpm for 30 s to give a porous film about 250 nmthick. After baking for residual solvent removal, the film undergoes twoconsecutive imprints both with FDTS treated mould 10. When the twoimprints are performed both at 200° C., 50 bar for 120 s or 300 s,parallelogram, diamond and square features result. The SEM images areshown in FIG. 6.

The pores in the imprinted regions collapse partially or completely sothat hybrid films with ordered arrays of porous domains result. Inaddition, there is little or no deformation in the resultanthierarchical structures. This difference may be attributed to the porousstructure of the HPI film, which can alleviate or dissipate stressesproduced during the imprinting process. The presence of pores enhancesthe first imprint taking place at a temperature below the Tg of thepolymer, as the imprint does not happen under the same conditions as onnon-porous HPI films.

Mechanism for Hierarchical NIL

In order to demonstrate the mechanism of the hierarchical patterning,Atomic Force Microscope (AFM) nanoindentatlon was conducted on plain PCfilm and protrusions of the 2 μm grating imprinted in the PC film. Theelastic modulus of the PC was independently determined to be 2.8 GPausing a Triboindenter. The elastic modulus of the imprinted sample wasevaluated using a comparison method reported by Wang et al. M. Wang, H.J. Jin, D. L. Kaplan and G. C. Rutledge, Macromolecules 2004, 37,6856-686:

$\begin{matrix}{\frac{S_{1}}{S_{2}} = \frac{r_{1}E_{1}}{r_{2}E_{2}}} & (1)\end{matrix}$

where S is the slope of the unloading curve at P_(max), P is the appliedload, r is radius of indentation contact and E is elastic modulus.Subscripts 1 and 2 denote imprinted and plain samples, respectively. Themethod measures the relative modulus, which is adequate for the presentpurposes.

The load versus displacement curves obtained for both samples are shownin FIG. 7, from which S and S are approximated to be 180 and 200,respectively. In FIG. 7 the dashed line is the approximated tangentialline at P_(max). AFM section profiles (FIG. 8) provide the contact sizes(“r” in eq. 1) of the Indents, which are 117 nm for the imprinted sampleand 61 nm for the pristine film, where A is the imprinted film and B isthe pristine film.

The modulus of imprinted PC was evaluated to be 1.3 GPa using eq. 1,which is about 50% lower than that for the pristine PC film (2.8 GPa).

The reduced elastic modulus is a result of the ‘flow’ of the polymer tofill in the trenches of the mould during the imprinting process. FIG. 9shows an illustration of this process. When a mould is pressed against apolymer film at above Tg, the polymer melt at immediate vicinity of themould protrusions flows into the mould trenches, which have a relativelylarge free space for the polymer chains to expand. At the same time, thepolymer melt at vicinity of the mould protrusions also becomes thinnerdue to outward polymer flow. As a result, the free volume of the polymerin the trenches of the mould is higher that that before the imprintingprocess. A higher free volume is believed to account for the lowermodulus in the protrusions of the imprinted polymer, thus allowing thesecondary or subsequent imprint to be carried out at a lower temperatureand pressure without the infusion of a plasticer such as an organicsolvent. The use of such plasticers is undesirable as it may causechanges to the properties of the polymer, and the surrounding componentand/or substrate.

Silicon grating moulds 10, 18 of 2 μm and 250 nm pitch respectively(both with 1:1 duty cycle) may be manufactured by photolithography andreactive ion etching. The moulds are preferably cleaned with acetone,isopropanol and oxygen plasma (80 W, 250 mTorr for 2 min). In the caseof imprinting on substrate-supported thin films, the moulds may befurther treated with perfluorodecyltrichlorosilane (FDTS, 20 mM inheptane) in a nitrogen glove box. The relative humidity may be kept at15% to 18%.

In hierarchical nanoimprinting of PC, all the primary imprints (i.e. 2μm grating) may be done at 180° C. or 160° C. and 40 bar; for PMMA, theprimary imprints were made at 120° C. and 40 bar.

Nanoindentations may be performed using MultiMode AFM (VeecoInstruments) with Nanoscope IV controller and silicon tip (RTESP model,nominal tip radius of curvature smaller than 10 nm). The spring constantand resonant frequency of the cantilever are 70 N/m and 259 kHz,respectively. The system may be force controlled, as opposed todisplacement controlled. The maximum applied load may be controlled to±1% or better accuracy by setting an appropriate trigger set point ofthe deflection signal. All indentations were made using the sameloading/unloading rate, 1 Hz. A 25° compensation of the probe during theindentation was used to prevent the cantilever from plowing the surfacelaterally, typically along the x direction. The AFM indentationprocedure consisted of three stages. First, the sample was inspected byAFM using tapping mode to locate the sample for indentation. Then AFMwas switched to force mode and the indentation performed. Finally, itwas switched back to tapping mode to image the indented area.

Whilst there has been described in the foregoing description preferredembodiments of the present invention, it will be understood by thoseskilled in the technology concerned that many variations ormodifications in details of design or construction may be made withoutdeparting from the present invention.

1-20. (canceled)
 21. A method for forming hierarchical patterns on anarticle by nanoimprinting, the method comprising: (a) using a firstmould to form a primary pattern on the article at a first temperatureand a first pressure, the first temperature and the first pressure beingable to reduce the elastic modulus of the article; (b) using a secondmould to form a second pattern on the primary pattern at a secondtemperature that is below the article's glass transition temperature,the forming of the second pattern being at a second pressure.
 22. Amethod as claimed in claim 21, wherein the first temperature andpressure are also able to reduce the melt viscosity of the article. 23.A method as claimed in claim 21, wherein the first temperature is abovethe article's glass transition temperature.
 24. A method as claimed inclaim 21, wherein the article is a polymer film.
 25. A method as claimedin claim 24, wherein the polymer film is selected from the groupconsisting of: thermoplastic polymers, polycarbonate,polymethylmethacrylate, porous polymers, and polyimide with hydroxy!side groups.
 26. A method as claimed in claim 24, wherein the polymerfilm is a polymer composite reinforced with particles selected from thegroup consisting of: calcium carbonate, carbon filler, glass filler,fibers, glass fibers, carbon fibers, and carbon nanotubes.
 27. A methodas claimed in claim 24, wherein the polymer film is on a substrate. 28.A method as claimed in claim 21, wherein the secondary pattern is formedat an angle relative to the primary pattern, the angle being in therange 0° to 90°.
 29. A method as claimed in claim 21, wherein at leastone of the first mould and the second mould is treated with a lowsurface energy coating for facilitating their release from the article.30. A method as claimed in claim 29, wherein the coating isperfluorodecyltrichlorosilane.
 31. A method as claimed in claim 21,wherein the first mould has a first grating for forming a first gratingstructure on the article, and the second mould has second grating forforming a second grating structure on the first grating structure.
 32. Amethod as claimed in claim 31, wherein the first grating is of a sizerelative to the second grating that is selected from the groupconsisting of: the same, larger, and smaller.
 33. A method as claimed inclaim 31, wherein the first grating and the second grating havedifferent geometries.
 34. A method as claimed in claim 31, wherein thefirst grating structure is protrusions and trenches on the article. 35.A method as claimed in claim 34 wherein the second grating is formed onthe protrusions and in the trenches.
 36. A method as claimed in claim31, wherein the first grating is of a 2 micrometers pitch and the secondgrating is of a 250 nanometers pitch.
 37. A method as claimed in claim21, wherein: (a) the first temperature is in the range of 1200° C. to200° C.; (b) the second temperature is in the range of 600° C. to 2000°C.; (c) the first pressure is in the range of 40 bar to 50 bar; and (d)the second pressure is in the range of 10 bar to 50 bar.
 38. A method asclaimed in claim 21, wherein before step (a) any residual solvent isremoved from the article.
 39. A method as claimed in claim 25, whereinthe polyimide with hydroxyl side groups is a porous film.
 40. A productproduced by the method of claim 21.